1. Field of the Invention
The present invention relates generally to the field of propulsion system aerodynamics and nacelle structures and, more specifically, to an improved bypass and exhaust system for front fan gas turbine engines for aircraft propulsion.
2. Description of the Related Art
In a turbofan gas turbine aircraft engine, a fan exhaust or engine bypass passageway, which terminates in a fan exhaust nozzle, is formed as an annular duct between an inner surface of the engine nacelle which may include a fan nacelle and fan reverser and an outer surface of the core engine cowling. Conventionally, the inner surface of the engine nacelle is centered on the engine center line and is circular in cross-section. The outer surface of the core engine cowling is also centered on the engine center line and is circular in cross-section, but the radius of curvature is smaller than that of the inner surface of the engine nacelle, thus defining the annular duct therebetween.
Increasing the size of the fan of a turbofan engine requires an increase in the size of the exhaust passageway and nozzle, due to the increased flow. Generally, it is desirable to maintain the annular shape of the fan exhaust passageway, particularly in the area of the trans-cowl, cascades reversing mechanisms, as well as the exhaust nozzle, so that operational loads create hoop stresses. Thus, in the past, increases in the size of the fan exhaust passageway and nozzle have resulted in an increase in the overall height of the engine, since in order to maintain an annular shape of the passageway the diameter of the engine must be increased symmetrically.
The motivation for increasing the size of the fan for a given engine is that higher by-pass, larger fan engines, which would require larger fan exhaust passageways and nozzle areas, are generally more efficient. However, the use of a symmetrically increased annular fan exhaust nozzle is limited in two important aspects: (1) since fan engines are mounted under the wing, there is a limit to the overall height of the engine, given that the height of the space between the underside of the wing and the ground is limited; and (2) increasing the overall height of the nozzle eventually creates wing interference drag.
U.S. Pat. No. 3,806,067 describes an asymmetric increase in the exhaust nozzle area achieved by vertically oblating the annular exhaust nozzle near the support pylon. While this vertical oblation results in an advantageous aerodynamic effect (e.g., reduced local static pressure upstream of the pylon which eliminates distortion effects on the fan), the increased overall height leads to the aforementioned problems.
U.S. Pat. No. 5,058,379 granted to Lardellier is directed to a tandem fan engine exhibiting an elliptical shape. A modest increase in bypass airflow is achieved with a lateral expansion of the nacelle. Non-circular pressure vessels such as the Lardellier nacelle which have non constant cross sectional radii have a tendency to distort into more circular shapes. Thick structural duct walls and strut supports are required to maintain the elliptical shape and maintain the airflows from the forward and aft fans separated. Thus, an expected benefit of lateral expansion to achieve greater bypass airflow is diminished by the heavy and bulky structural supports required to implement the design.
There thus exists an unfulfilled need for a lightweight aerodynamic and structurally sound elongated nacelle having an increased exhaust passageway for increased bypass airflow.